We investigate subwavelength confinement of terahertz electromagnetic surface modes in a three-dimensional
region with coupled slot structures. Two-dimensional resonance focusing on a subwavelength slot converts to
three-dimensional subwavelength confinement, due to sharp edge confinement effect on asymmetric
plasmonic structure, at the center position of the slot structures which consists of two or more slots. We also
report on the polarization independent confinement of terahertz electromagnetic surface modes beyond
diffraction limit. The structure which consists of radially arranged subwavelength slots located at a same
center position shows the polarization-independent terahertz three-dimensional subwavelength confinement.